1. Field of the Invention
The present invention relates to an information reproducing apparatus such as a magneto-optic disc apparatus, and a reproducing method.
2. Description of the Related Art
In an information reproducing apparatus such as a magneto-optic disc apparatus, generally, for causing a PLL (Phase Locked Loop) to be locked with a reproduced signal reproduced from a record medium, clock pulses are generated and a reproducing system is operated corresponding to that clock pulses. Therefore, it is necessary that clock pulses including no phase error or sufficiently small phase error is produced for causing a PLL to be locked adequately, so as to reproduce recorded information from record medium correctly.
It is necessary that both an acquisition mode and a tracking mode are operated adequately so as to cause a PLL constantly to be locked adequately.
When a PLL has not been caused to be locked, the PLL is caused to be locked with an acquisition mode. On the other hand, when a PLL has already caused to be locked, PLL is held to be locked with a tracking mode. A frequency lock mode has been used as an acquisition mode, and a Viterbi determination mode has been used as a tracking mode up to this time.
In a frequency lock mode, predetermined signal pattern has been recorded or formed on the predetermined area of a record media. And a PLL is caused to be locked with that predetermined signal. For example, the 5.25 inch magneto-optic disc that is up to the standardization of ISO (International Organization of Standardization), 2T (T:bit period)pattern is used as the predetermined pattern. A frequency lock mode is suitable for an acquisition mode.
On the other hand, a Viterbi determination mode can be adopted to an information reproducing apparatus that performs a Viterbi decoding process.
In a Viterbi determination mode, phase error is detected with status data, namely series of status data value that represents a status transition selected in a Viterbi decoding process. Then it is necessary that the Viterbi decoder that generates status data is used for a Viterbi determination mode to be performed. A Viterbi determination mode is suitable for a tracking mode.
A frequency lock mode and an acquisition mode have merits and demerits respectively. Therefore, PLL can be caused to be locked more stably with a structure in which these two modes are shifted over one to another in adequate timing corresponding to circumstances of a PLL. For, that structure can make up the demerits that these two modes respectively has.